During drilling and production operations, it may be necessary to inspect the structural integrity of casing that lines a wellbore. Casing integrity can be checked by imaging the casing to detect defects such as corrosion, and remedial measures can be taken before the casing is damaged because of these defects. Typically, the imaging is carried out using electromagnetic or acoustic systems.
An electromagnetic high frequency system, however, does not have good detecting capability when probing the interior of the casing or beyond a casing or tubing liner because the material of the casing (e.g., steel) is too lossy for electromagnetic (EM) waves to penetrate deep into the casing. A low-frequency EM system may afford enough penetration capability, but the resolution is low and the system is not sensitive enough to small changes in the structure of the casing due to corrosion.
In some cases, an acoustic system is used instead of an electromagnetic system since the acoustic system offers high image resolution. However, acoustic measurements from an acoustic system are easily deteriorated due to background mechanical vibration. The coupling of acoustic energy into the target is also difficult if the acoustic impedance mismatch is large. As a result, a major portion of the incident acoustic energy is reflected from the target boundary and only a small amount of the incident acoustic energy penetrates into the interior of the target.